Best way to beat that mission is to follow this strategy.
Here is a strategy by redalert19243 on the DF message boards:
"The following is a fairly successful strategy I used to beat the
level..... I couldn't beat it without letting the Bastion's hull drop
below 35..... When I used this strategy the Bastion's hull remained
somewhere around 70 (which is excellent) and I really had no trouble at
all.
First, make all of Alpha wing Valkyries (including yourself) since they
have a high top speed and acceleration, which is ideal for intercepting
bombers, and equip them with Banshee and Prometheus cannons since
Ulysses can't use them. Give them Hornet missiles, but give yourself
interceptors (you'll see later). Assign Alpha wing to protect the
Bastion. Make all of Beta wing Ursa bombers and equip them with
Harbingers, Synaptics (for defense) and Stillettos. Have them attack
the Tantalus while you provide cover for the Bastion (sounds like
suicide). Your job is to target all of the bombers first (VERY
IMPORTANT) before they can reach the Bastion. The ones to look out for
are the Nephilims (initially) and Shaitans (later). A helpful way of
protecting the Bastion is to target all of the megabombs being
delivered if you can't intercept all of the bombers in time. Target
them with T since hostile (H) targeting won't pick them up. Use the
intercepts that you equipped to target the bombs since they have a
faster lock and velocity. Once you cleaned them up, go for rest of the
packs, especially Basilisks since they have nasty swarm bombs. When
your done with them, the Tantalus should be very low on health and feel
free to destroy it with whatever remaining fighters you have. In a
moment the Bastion will jump out and the mission will be complete."
